The present invention relates to a process for the preparation of .alpha.,.omega.-C.sub.6 - to C.sub.20 -alkenols of high purity by the catalytic dehydration of the corresponding .alpha.,.omega.-C.sub.6 - to C.sub.20 -diols.
The dehydration of alcohols is a conventional method, which is also utilized technically and industrially, to prepare olefins and/or generally to introduce a double bond into a molecule. (See, e.g., "Ullmanns Enzyklopaedie der technischen Chemi" [Ullmann's Encyclopedia of Technical Chemistry] 10 [1958]: 41 and 8 [1957]: 695 and 696.) It is furthermore known that such dehydrations can be conducted most easily with tertiary alcohols. The reaction is more difficult for secondary alcohols and can be effected only under extreme conditions with primary alcohols.
Moreover, in such dehydrations in the liquid as well as gaseous phases at temperatures above 350.degree. C. in the presence of acidic catalysts and in the presence of solid catalysts, a shift of the position of the double bond occurs to a partial extent. With increasing reaction temperatures, this positional shift occurs to an increasing extent. If it is desired to produce uniform products free of isomers, the resultant positional isomers in the reaction product represents a grave problem. Therefore, various methods have been suggested to prevent isomerization. However, when interpreting the older treatises, it must be borne in mind that often the analytical methods available at the time of the writing frequently were not capable of the exact characterization of the reaction products. (See, e.g., Houben-Weyl, "Methoden der organischen Chemie" [Methods of Organic Chemistry] V/1b "Alkenes, Cycloalkenes, Arylalkenes" [1972]: 45 et seq.)
The actual extent of isomerization can only be determined by means of exact analytical methods, for example, by gas chromatographic analyses. Literature data which are not based on such accurate analytical methods cannot reliably indicate anything regarding the selectivity of the reaction. Since the isomerization is catalyzed by acids or by acidic centers on the solid catalysts, it occurs especially during the dehydration of primary alcohols requiring drastic conditions for the elimination of water, i.e., strong acids and high temperatures. All of the conventional processes for dehydrating alcohols while minimizing isomerization, thus, have as an objective the maximum avoidance of strong acids and relatively high reaction temperatures during the reaction, and of at least partially neutralizing the acidic centers with bases when using solid catalysts. Suitable bases for ameliorating the acidic character of a catalyst include, for example, nitrogen compounds, such as ammonia, tertiary amines, pyridines, etc., and also sodium hydroxide solution and sodium carbonate solution.
Such partial neutralization of solid catalysts, however, also entails grave disadvantages. Although the suppression of isomerization is more effective, the more basic is the compound employed to lessen acidity; concomitantly, the higher is the reduction in activity of the catalyst. The higher purity of the reaction product must, accordingly, be attained at a cost of long reaction periods. An additional disadvantage of these partially neutralized catalysts is the shorter resultant catalyst lifetime.
The dehydration of amyl alcohol in the presence of tricalcium phosphate at 440.degree. C., for example, yields 86.9% of pentenes with a 1-pentene content of 20.2%. After treating the catalyst with sodium hydroxide solution, the 1-pentene content does rise to 97.2%, but the yield in pentenes decreases to 74.6%. Furthermore, the catalyst treated with sodium hydroxide solution must be are generated after only 100 hours, since its activity has been excessively reduced (Chem. Abstr. 58: 2353 [1963]). Such processes having catalyst operating periods of only a few days are uneconomical and thus are not implemented on an industrial scale.
All of the known processes, therefore, exhibit considerable disadvantages, in that they either result in nonuniform products using relating active catalysts at high space-time yields, or yield pure substances using partially neutralized catalysts in an uneconomical mode of operation.